There are several types of wireless optical data transmission systems. One possibility for realising such a wireless system for transmitting data is using white light emitting devices that are pulsed at a high frequency, which could not be detected by the human eye. Modulation scheme of on/off keying (OOK) for modulation is adopted. It is possible to use different colours of light emitted by a multi-chip-type white LED as parallel transmission channels.
The white light of multi-chip-type white LEDs is produced by simultaneously emitting light in the three basic colours, blue, red and green. The superposition thereof results in white light. The problem with using multi-chip-type white LEDs for transmitting data in parallel is that the power ratio of each light emitting device has to be different in order to obtain white light. When a multi-chip-type white LED is used for illumination it is not acceptable to emit bluish or reddish light for example. Thus, the power ratio of the emitted light of the three colour LEDs is fixed and cannot be adjusted.
The emitted light of the three LEDs is detected by three separate detectors, but the conversion efficiency of each of the detectors depends on the colour of each of the emitting LEDs. Therefore, the reliability of the output signal of the detectors is different for each colour.
For assuring an error rate which is less than 1E-10 e.g. for wireless communications, an error control decoder is required. It is known that the performance of soft decision error control decoders is usually better than the performance of hard decision error control decoders. But even the use of a soft decision error control decoder might not achieve the required error rate, because of the different reliability on each colour.
It is therefore an object of the present invention to create a method for wireless optical transmission of data and a wireless optical transmission system to improve the performance of soft decision error control decoding.
This problem is solved by a method according to claim 1 and a data transmitting system according to claim 10.